1. Field of the Invention
This invention relates to optical systems comprising two elements suitable for use in photographic devices having curved image surfaces.
2. Description of the Prior Art
U.S. Pat. No. 3,006,248 by W. R. Linke (issued in 1961) discloses an objective for cameras having a cylindrical image surface that comprises two meniscus lens elements. The disclosed optical system, "while of the symmetrical type is not completely symmetrical". It is basically a rear landscape lens with a meniscus corrector lens in front. The power of the front lens is weaker than the power of the second lens.
Other proposals have been made for optical systems (or lens systems) including two different asymmetrically located lens elements. For example, U.S. Pat. Nos. 4,932,764 and 5,000,552 disclose a two-element lens system suitable for use in inexpensive cameras, such as single-use cameras. This lens system utilizes two meniscus lens elements arranged around the aperture stop. The lens system is characterized by very good aberration correction, considering that only two lens elements are used.
U.S. Pat. No. 5,067,803 also discloses a two-element lens system for use in photographic cameras. This patent also uses two meniscus lens elements arranged around a central aperture stop.
The two lens element camera objectives are often designed to provide an F/number of F/11. Although it is advantageous to have a faster lens system, that is difficult to accomplish with only two elements.
In addition, the assembly of these types of lens systems is relatively simple, but has one complication. That is, if the lens elements are mounted into two separate cells (or units) with an aperture stop in between, they have to be centered with respect to each other. A badly mounted lens element that is sensitive to decenter will introduce aberrations into the optical system and thus adversely effect the image quality. Therefore, the cells containing the lens elements have to be carefully aligned with each other. This complexity increases manufacturing costs.
Applicants' invention represents a further improvement in two element lens systems.